The conventional electroconductive contact probe used for electrically testing conductive patterns of printed circuit boards and electronic devices normally comprises an electroconductive needle member, a tubular receptacle slidably receiving the needle member, and a spring member biasing the needle member out of the receptacle from its free end so that the free end of the needle member may be elastically pushed against an object to be tested. Such electroconductive contact probes are disclosed in U.S. Pat. Nos. 5,003,255 and 5,004,977 both issued to Kazama Mar. 2, 1991 and Apr. 2, 1991, respectively, and copending patent applications Ser. Nos. 07/737,763 and 07/739,051 both filed Jul. 30, 1991. The contents of these patents and patent applications are incorporated in the present application by reference.
In a contact probe unit incorporating a multiplicity of such contact probes for simultaneously accessing a multiplicity of points, the receptacles of the individual contact probes are secured to an electrically insulating receptacle holder in a mutually parallel relationship. With the demand for an increasingly higher wiring density in the recent printed circuit board design, 1% has become increasingly important to arrange the individual contact probes as close to each other as possible in the receptacle holder.
FIG. 4 illustrates a conventional contact probe unit for accessing a multiplicity of points in which the free ends of the individual contact probes are arranged in a highly dense distribution. Each individual contact probe 22 is passed through an electrically insulating receptacle holder 21 and fixedly secured thereto. An electrically conductive needle member 23 is slidably received in each of the receptacles 22. A compression coil spring not shown in the drawing is received in each of the receptacles 22 for urging the needle member out of the free end (the lower end as seen in FIG. 4) of the receptacle 22.
The contact probe unit further includes a free end guide plate member 25 which is secured to the receptacle holder 21 in a mutually parallel relationship by support posts 24. The free end guide plate member 25 is provided with a multiplicity of guide holes 25a each receiving the free end of the needle member 23 of the corresponding contact probe in a freely slidably manner. The density of the distribution of the guide holes 25a in the free end guide plate member 25 is higher than the density of the contact probes 22 in the receptacle holder 21. The base end of each of the needle members 23 received in the rear end of the corresponding receptacle 22 is connected to a control unit not shown in the drawing by way of a lead wire 26. Thus, the contact probe unit can simultaneously access a multiplicity of points of a highly densely arranged electroconductive pattern 27a of a printed circuit board 27 even when the density of the points to be accessed is higher than the highest possible density with which the contact probes may be mounted on the receptacle holder of the contact probe unit.
However, according to such a conventional contact probe unit, since the needle members 23 are not assisted by any member in the space between the receptacle holder 21 and the free end guide plate member 25, it is highly difficult to pass the individual needle members through the corresponding guide holes of the free end guide plate member 25 during the process of fabricating or repairing each contact probe unit. In particular, when the contact probes ere arranged in a two dimensional layout, the efforts required for the assembly work and the maintenance work are significant.
Further, each of the needle members 23 is subjected to a substantial axial compressive stress when it is pushed against a point to be tested by overcoming the spring force, there is a tendency to buckle in the part of the needle member 23 situated between the receptacle holder 21 and the free end guide plate member 25 as indicated by the imaginary lines in FIG. 4. If such a buckling of the needle members is excessively, the adjacent needle members 23 may touch each other. An additional problem is the difficulty in achieving a uniformity of the axial positions of the free ends of the needle members 23.